The invention pertains to a uniform composite in a hypermonotectic alloy system and a method of producing the same. More specifically, the invention pertains to a uniform composite in a hypermonotectic alloy system and a method for producing the same wherein a plurality of constrained fibers, aligned paralled to the direction of growth, are used to provide for the directional solidification of L.sub.II phase.
Alloys of hypermonotectic composition are used and have been suggested for application as bearing materials, free machining materials, electrical materials, electrical contacts, glasses and superconductors. Therefore, these alloys have useful applications.
The solidification of hypermonotectic alloys involves passing through a miscibility gap, which is a characteristic of a hypermonotectic alloy. In this gap, two liquids, often of very different composition and density, co-exist in the liquid state. This density difference promotes rapid phase separation, and hence, massive segregation upon solidification. Massive segregation is an undesirable result because it leads to inferior material properties.
Others have postulated that sedimentation could be eliminated by processing these alloys in a microgravity environment so as to achieve a uniform composite structure. Although the microgravity concept generated considerable interest, it was found that processing in the microgravity environment still produced massively segregated structures. A number of explanations for these results have been proposed that include phase diagram inaccuracy, insufficient mixing, residual or induced convection, thermal gradient effects, droplet coalescence, and preferential wetting of the container by one of the liquid phases.
One aim of the present invention is to eliminate phase diagram inaccuracy, insufficient mixing, residual and induced convection, thermal gradient effects as limiting factors. Another aim is to use droplet coalescence, and preferential wetting of the container by one of the liquid phases to help achieve a uniform structure without macrosegregation.
The binary miscibility gap systems of interest are characterized by a region where two distinctly different liquids are in thermodynamic equilibrium, and the presence of the monotectic reaction, L.sub.1 =S.sub.I +L.sub.II. Though similar in form to the well studied eutectic, two possibilities exist at the monotectic reaction. These possibilities are that L.sub.II can either wet or not wet S.sub.1.
In the first case where L.sub.II wets S.sub.I, and when employing controlled directional solidification techniques, the microstructure of the former case exhibits a uniform, hexagonal close-packed array of aligned L.sub.II fibers. In the second case when L.sub.II does not wet SI, and when using controlled directional solidification techniques, L.sub.II droplets collecting at the interface are pushed and eventually physically incorporated, leading to a somewhat irregular structure. This has been theoretically discussed by Chadwick (G. A. Chadwick, Brit.J.Appl.Phys., 1965, Vol. 16, pp. 1095-1097) and Cahn [J. W. Cahn, Metall.Trans.A., 1970, Vol. 10A, pp. 119-121]and experimentally verified by Livingston and Cline (J. D. Livingston and H. E. Cline, Trans.TMS-AIME, 1969, Vol. 245, pp. 351-357) and Grugel and Hellawell (R. N. Grugel and A. Hellawell, Metall.Trans.A., 1982, Vol. 12A, pp. 669-681). Furthermore, the transition from wetting to non-wetting has been shown to change abruptly through the selected addition of a ternary component, both in organic [M. R. Moldover and J. W. Cahn, Science, 1980, Vol. 207, pp. 1075-1076; and R. N. Grugel and A. Hellawell, Materials Research Soc. Proc., Elsevier Science Publishing Co., Inc., 1982, Vol. 19, pp. 417-422) and metal (R. N. Grugel and A. Hellawell, Metall.Trans.A.. 1982, Vol. 12A, pp. 669-681) systems.
Uniform solidification of hypermonotectic alloys has been hampered by the inherent, usually large, density differences between the L.sub.I and L.sub.II phases. This leads to rapid separation, coalescence and, consequently, a highly inhomogeneous structure. Although some inhomogeneity can be minimized by employing rapid solidification techniques (S. N. Tewari, J. Mat. Sci., 1989, Vol. 8, pp. 1098-1100. J. L. Reger, Proc. 3rd Space Processing Symposium Skylab Results: Vol. 1, NASA Report M-74-5, 1974, pp. 133-158), there is a great need to be able to produce a uniform composite in a hypermonotectic alloy system.